Traditionally, the amplifier using an operational amplifier is well known as a means that amplifies a minute analog signal such as a microphone input signal. There are an inverted amplifier and a non-inverted amplifier as the amplifier using an operational amplifier, which is well known.
In the inverted amplifier, the inverted input terminal of the operational amplifier inputs a minute analog signal through an input resistor. The non-inverted input terminal of this operational amplifier inputs a signal ground potential. The output terminal and the inverted input terminal of this operational amplifier are connected through a feedback resister.
In the non-inverted amplifier, the inverted input terminal of the operational amplifier inputs a signal ground potential through an input resister. The non-inverted input terminal of this operational amplifier inputs the minute analog signal. The output terminal and the inverted input terminal of this operational amplifier are connected through a feedback resister.
If a very large gain is desired, it is only needed to cascade two or more inverted amplifiers or non-inverted amplifiers.
As mentioned above, in the amplifier using an operational amplifier (inverted amplifier or non-inverted amplifier), one of the inverted input terminal or the non-inverted input terminal inputs a minute analog signal, and the other one inputs a signal ground potential. And, the difference of the minute analog signal and the signal ground potential is amplified. Accordingly, if the signal ground potential has noises superposed, this operational amplifier amplifies the noises as well.
And, the minute analog signal such as a microphone input signal is generated by means of a signal generator provided in the pre-stage of the amplifier. Normally, this signal generator generates the signal by using a signal ground line different from what is used for the amplifier. When the signal ground potential of the signal generator has noises superposed, the output of this signal generator (namely, the minute analog signal) has the noises superposed as well. These noises are also amplified through the operational amplifier of the amplifier, in the same manner as the noises of the amplifier.
In order to ensure a sufficient quality of the minute analog signal, the amplifier is preferably configured not to amplify the noises on the signal ground line of the amplifier and the signal generator.